Eddy current and magnetic methods are two common techniques for non destructive evaluation (NDE) of materials and parts. Technology in present use seriously limits range of application, because existing sensors lack either sensitivity, bandwidth, spatial resolution, or combinations of these properties. For example, eddy current systems typically use induction coil receivers to detect the magnetic field of oscillating eddy currents induced in a metallic object. Because signal to noise varies linearly with frequency, they are constrained to operate at frequencies from tens of kHz to a few MHz. In a conductor, induced currents are exponentially attenuated with depth below the surface, according to the skin depth .delta.=.sqroot.1/.pi..mu..sub.o .sigma..function., where .mu..sub.o is the permeability of vacuum, .sigma. is the material's conductivity, and .function. is the oscillation frequency. For aluminum alloys, .delta..apprxeq.1 mm at 10 kHz. Conventional systems are insensitive to flaws buried deeper than one or two skin depths below the surface. Furthermore, induction coil receivers require a large turns-area product to attain adequate sensitivity, sacrificing spatial resolution in the process. They fail to meet simultaneously all three requirements of sensitivity, penetration depth, and resolution.
A need has long existed to provide improved sensitivity, penetration depth and spatial resolution in non destructive evaluation of flaws within conductors and magnetic materials.